From DE 10 2012 218 876 A1 an actuating device for a friction clutch is known, having a clutch cover, a diaphragm spring supported directly or indirectly on the clutch cover so that it can swivel around a swivel area, in particular ring-shaped, to move a contact plate to press a clutch disk between the contact plate and a counter plate, a bearing spring supported directly or indirectly on the clutch cover to provide a counter bearing to disperse the forces of the diaphragm spring occurring at the swivel area, and a servo spring, in particular in the form of a diaphragm spring, positioned between the bearing spring and the diaphragm spring in the axial direction, the diaphragm spring and the servo spring being in contact with one another in at least two contact zones which are spaced apart from one another in the radial direction.
The servo spring reduces the consequences of a known effect, that with a diaphragm spring the necessary actuating force to swivel the diaphragm spring in order to move a contact plate increases sharply when the diaphragm spring rises. When friction linings provided between the contact plate and the counter plate to transfer torque frictionally to the clutch disk are in new condition, the contact plate travel distance to engage the friction clutch is still very short, so that the diaphragm spring cannot rise significantly. When the friction linings wear, the thickness of the friction linings decreases, which forces the contact plate to traverse a greater distance in order to engage the friction clutch and produce a frictional engagement with the clutch disk. This makes it necessary for the diaphragm spring to traverse a longer swivel distance so that the diaphragm spring is able to rise through elastic deformation, and accordingly the spring characteristic provided for the diaphragm spring would make an increased actuating force necessary to move the contact plate. This increased actuating force is counteracted by the servo spring. Thus, the servo spring ensures a low actuating force to move the contact plate, and an adequate contact force when engaging the friction clutch, over the life of the friction clutch.
From WO 2014/056495 A1 a friction clutch device is known, in particular for a drivetrain of a motor vehicle propelled by an internal combustion engine, having an axis of rotation, a housing, at least one contact plate which is movable relative to the housing in the direction of the axis of rotation for actuation between an engaged actuation position and a disengaged actuation position, a first spring to load the at least one contact plate and a second spring designated as a servo spring to reduce an actuating force in a locked state, the second spring being supported directly or indirectly at a first diameter on the housing and directly or indirectly at a second diameter on the first spring, the first diameter being smaller than the second diameter.
From WO 2015/010694 A2 a friction clutch device is known, in particular for a drivetrain of a motor vehicle propelled by an internal combustion engine, having an axis of rotation, a housing, at least one contact plate which is movable relative to the housing in the direction of extension of the axis of rotation for actuation between an engaged actuation position and a disengaged actuation position, a first spring to load the at least one contact plate and a second spring designated as a servo spring to reduce an actuating force, the second spring being supported with a first support radius and with a second support radius on the housing, and additionally on the first spring.